Hi maggamuff,
This basically means they only need one air traffic controller at the moment, they aren't hiring new trainees, they want someone who is already qualified. To get this qualification is normally only possible by being hired by an air traffic control company who will train you to get a ACS/RAD license (you would also then have to work for that company for a minimum number of years before you can leave). Sorry that this does not help so much, you will have to look for other countries who are hiring trainees at the moment, otherwise wait and see what happens in Croatia control!